Product designers continually seek ways of reducing the time and cost of the product development cycle. Prototyping is often used during product development in order to verify design concepts and facilitate mechanical testing. A prototype must have characteristics sufficiently close to the desired product to permit a realistic prediction of actual product performance. Products that have injection-molded plastic components can be expensive to prototype and require a long lead time. These injection-molded plastic parts require a metal mold in order to create prototypes that are dimensionally accurate. The metal mold is expensive to produce and requires a long lead time, thereby necessitating a significant allocation of time to designing the mold, machining the mold cavity, texturing the surface of the mold, and so forth. Typically, several design iterations are required for each new product, further increasing the high cost and long lead time. Thus, it is clear that metal mold techniques are not well suited for rapid prototyping applications.
One alternative to the metal mold has been low-cost silicone rubber molds. A typical method of preparing this mold involves creating a model of the article or part to be molded. Pourable silicone rubber is employed to form a three-dimensional mold of the article to be copied. These liquid silicones are generally poured over a portion of the article and then cured to form one-half of the mold. The process is then repeated with the other half of the article, creating the second half of the mold. The two mold halves are then mated together and the empty cavity representative of the article is filled by pouring a material, such as epoxy, into the mold. After the epoxy has cured, the two mold halves are separated and the molded article is removed. The use of a flexible rubber for the mold allows production of articles having considerable backdraft portions, and also reduces the cost and the cycle time required to create prototypes.
However, fundamental problems remain. For example, the number of materials useful for casting the molded article is limited to those which are easily pourable at or near room temperature. Further, the casting technique is limited with regard to the quality of the finished part that can be created--air voids and other dimensional inaccuracies are inherent to the gravity-poured casting technique. Another fundamental problem is distortion of the flexible mold by the poured material, either as a result of the forces applied from the sheer mass of the epoxy, or due to expansion or contraction of the material during the curing cycle. Although the distortion of the mold may be relatively slight, it becomes significant with high-precision or thin-walled parts, and is unacceptable. Further, pourable materials such as epoxy require a long cure time, severely limiting the number of parts that can be produced in a day. The long cycle time for molding each part makes the production of a reasonable number of parts difficult and expensive. Furthermore, filling of the mold with the resin when using the gravity casting technique is awkward and difficult.
Clearly, a low-cost method for rapid prototyping low to moderate volume production of precision plastic articles is needed. Current techniques for prototyping molded parts are costly and inadequate for most applications. A molding technique that permits the utilization of a wider number of plastic materials more suitable to that used in the end product would be desirable. Hence, there is a need for a more expeditious and cost-effective way of producing dimensionally accurate, precision-molded parts.